The chemical reaction of chlorine with alkylbenzenes, such as toluene, to prepare nuclear substitued chloro-compounds such as monochlorotoluene, is well known and of considerable commercial importance. Such reactions are generally carried out in the presence of a chlorination catalyst such as antimony chloride, ferric chloride, aluminum chloride, and the like. The usual products of such reactions are a mixture of various mono-chlorinated and/or polychlorinated compounds and various positional isomers of these. For example, in the liquid phase substitution-chlorination of toluene, by reaction of chlorine and toluene, to form monochlorotoluene, the usual product is a mixture of orthochlorotoluene and parachlorotoluene which may, in addition, contain varying amounts of other chlorinated products such as metachlorotoluene, dichlorotoluene, polychlorotoluenes and benzylic chlorides. Of the major reaction products, that is orthochlorotoluene and parachlorotoluene, the latter is the most commercially valuable. In the past, considerable effort has been expended in attempts to direct the chlorination reaction in such a manner as to lower the ratio of orthochlorotoluene to parachlorotoluene, that is, to discover reaction conditions under which the formation of parachlorotoluene is favored. Thus, for example, it is known from U.S. Pat. No. 1,946,040 that when alkylbenzenes are reacted with chlorine, the yield of parachlorinated product is improved with the aid of a mixed catalyst comprising sulfur and antimony trichloride and, optionally, iron or lead. In British Patent 1,153,746 (1969) it is disclosed that in the chlorination of toluene in the presence of a ring chlorination catalyst, such as ferric chloride, antimony chloride, and the like, the ratio of orthochloro to parachloro isomers produced may be lowered by the presence of an organic sulfur compound such as thiophene, hexadecylmercaptan, dibenzothiophene or the like. Furthermore, in British Patent 1,163,927 (1969) it is disclosed that the proportion of parachlorotoluene produced may be improved when toluene is chlorinated in the presence of elemental sulfur or an inorganic sulfur compound and a ring-chlorination catalyst such as ferric chloride, aluminum chloride, antimony chloride, zinc chloride, iodine, molybdenum chloride, stannous chloride, zirconium tetrachloride or boron trifluoride. In U.S. Pat. No. 3,226,447, issued December 28, 1965 to Bing et al, it is disclosed that in the substitution-chlorination of benzene and toluene, the ratio of ortho isomer to para isomer in the chlorinated product may be lowered when the reaction is carried out in the presence of an iron, aluminum or antimony halide catalyst and a co-catalyst which is an organic sulfur compound wherein the sulfur is divalent. Examples of such co-catalyst include various mercaptans, mercapto-aliphatic carboxylic acids, aliphatic thiocarboxylic acids, alkyl sulfides, alkyl disulfides, thiophenols, aryl sulfides, aryl disulfides and the like containing divalent sulfur. The use of such co-catalysts in the chlorination of toluene produces a product wherein the ratio of orthochlorotoluene to parachlorotoluene is 1.2, indicating a considerable improvement over the ortho to para isomer ratio achieved in the absence of the co-catalyst. However, it will be apparent that even a 1.2, ratio of ortho to para isomer represents a considerable economic disadvantage in the production of substantial amounts -- greater than 50 percent of the monochlorotoluene mixture -- of the unwanted ortho isomer. Thus, it will be apparent that a considerable commercial benefit is to be derived from a still further lowering of the ortho to para isomer ratio.
Still further improvements in the preparation of monochlorotoluene having a low ortho to para isomer ratio are disclosed in co-pending applications Ser. Nos. 601,219 and 601,690 to John C. Graham, now U.S. Pat. Nos. 4,031,147 and 4,031,142, respectively. Co-pending application Ser. No. 601,690 discloses a process for the preparation of nuclear chlorinated alkylbenzenes, such as monochlorotoluene which comprises reacting an alkylbenzene, such as toluene, with chlorine in the presence of a Lewis acid catalyst and, as a co-catalyst, thianthrene. When toluene is chlorinated in accordance with the process disclosed in co-pending application Ser. No. 601, 690, a monochlorotoluene product having an ortho to para isomer ratio of about 1.0 is obtainable.
In accordance with co-pending application Ser. No. 601,219, a monochlorotoluene product having an ortho to para isomer ratio of less than about 1.0 is obtainable with the aid of a co-catalyst comprising a thianthrene compound having electron-withdrawing substituents, such as chlorine, present on the nucleus thereof. Thus, in accordance with co-pending application Ser. No. 601,219, an alkylbenzene is reacted with chlorine in the presence of a Lewis acid catalyst and a co-catalyst comprising a thianthrene compound, or mixture of thianthrene compounds, characterized by the formula: ##STR2## where each n is 0 to 1, and each x is hydrogen or an electron-withdrawing substituent.
Although the processes of co-pending application Ser. Nos. 601,219 and 601,690 provide superior results to the prior art discussed above it will be apparent that still further improvements would be desirable and of commercial benefit. Furthermore, the co-catalyst, especially the chlorinated thianthrene cocatalyst of Ser. No. 601,219 is synthesized by a two-step reaction from a specific and limited selection of raw materials. Thus, it will be seen that an advantage is to be derived from the use of a co-catalyst that may be more easily synthesized from readily available raw materials.
It is an object of the present invention to provide an improved process for the directed nuclear chlorination of aromatic compounds. It is a further object to provide a process for the directed nuclear chlorination of alkylbenzens, especially toluene, whereby the chlorinated product is characterized by a desirably low ratio of orthochloro to parachloro isomers. It is a still further object to provide an improved para-directing co-catalyst for such processes, that may be conveniently synthesized from readily available raw materials. It is a still further object to provide a novel catalyst system based on a para-directing co-catalyst comprising a thianthrene compound or mixture of thianthrene compounds, having both electron-withdrawing substituents and electron-donating substituents on the nucleus thereof.
The thianthrene compounds employed as para-directing co-catalysts in accordance with this invention are described hereinbelow in accordance with the current Chemical Abstracts system whereby the numbering of ring positions is as follows: ##STR3##